1. Field of the Invention
The present invention relates in general to an engine, and more particularly to an exhaust system for a marine engine.
2. Description of Related Art
Personal watercraft have become popular in recent years. This type of watercraft is sporty in nature; it turns swiftly, is easily maneuverable, and accelerates quickly. Personal watercraft today commonly carry one driver and one or two passengers and include a hull which defines an interior engine compartment for housing an engine. The output shaft of the engine is coupled to a water propulsion device of the watercraft, such as a jet propulsion unit. An internal combustion engine is commonly used to power the personal watercraft. Typically, the engine is an in-line, multi-cylinder, two-cycle engine.
An exhaust system communicates with the cylinders of the engine and extends to a discharge that is located near the stem of the watercraft. In this manner, exhaust gases are expelled from the watercraft. To improve engine performance, the engine exhaust system includes an expansion chamber that has a larger diameter than the exhaust pipe in which it is located. The expansion chamber serves to improve engine efficiency by creating return shock waves that regulate the exhaustion of gases out of the compression cylinders following combustion. The expansion chamber often includes a diffuser cone and a convergent cone, both of which provide a size transition from the smaller diameter exhaust pipe to and from the expansion chamber.
Because the exhaust gases may cause air and/or water pollution, a catalyst comprising suitable catalytic materials is housed within the exhaust gas passage of the exhaust system to reduce harmful emissions. The effectiveness of the catalytic materials generally increases with the volume of the catalyst. It has been previously proposed to house the catalytic materials in the expansion chamber. However, housing the catalyst in the expansion chamber reduces the net space of the expansion chamber, which reduces the advantage of having an expansion chamber. The problem is that the catalyst interferes with the force and speed of the return shock wave created by the passage of exhaust gases through the diffuser and convergent cones within the expansion chamber. Formation of the return shock wave enhances performance of the two-stroke engine and the catalyst may adversely inhibit return shock wave formation.
A need, therefore, exists to improve the structure of the exhaust system that will not deteriorate the engine performance while maintaining the catalyst within the exhaust system.